Electro Luminescence Display and Method of Testing the Same

ABSTRACT

To increase the proportion of the perfects to the whole lot of final products and to reduce the cost for active matrix EL display devices by checking the operation of a TFT substrate before depositing an EL material. A capacitor for testing is connected to a drain terminal of a driving TFT in a pixel portion to observe charging and discharging of the capacitor. Whether the driving TFT is normal or not is judged by the observation, so that the rejects can be removed before the manufacturing process is completed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronic display (electro opticaldevice) formed by fabricating an EL (electro luminescence) on asubstrate. In particular, the present invention relates to a displaydevice using a semiconductor element (an element which uses asemiconductor thin film). Further, the present invention relates to anelectronic device using an EL display in a display portion and themethod of detecting the EL display.

The EL element herein refers to both an element that utilizes lightemission from a singlet exciton (fluorescence) and an element thatutilizes light emission from a triplet exciton (phosphorescence).

2. Description of the Related Art

Recently, a technique for forming a thin film transistor (hereinafter,referred to as TFT) on a substrate has been remarkably developed, and adevelopment of its application to an active matrix display device hasbeen continuously made. In particular, TFTs using a polysilicon film canoperate at high speed, because such TFTs have a higher field effectmobility than TFTs using a conventional amorphous silicon film.Therefore, the control of pixels, which has been conventionallyconducted by a driver circuit provided outside a substrate, can beperformed by a driver circuit provided on the same substrate on whichthe pixels are provided.

Such an active matrix display device includes various circuits andelements formed on the same substrate. With this structure, the activematrix display device provides various advantages such as reducedmanufacturing cost, reduced size of a display device, an increasedyield, and a reduced throughput.

Furthermore, an active matrix EL display device including an EL elementas a self-luminescent element has been actively studied. The EL displaydevice is also called Organic EL Display (OELD) or Organic LightEmitting Diode (OLED).

In contrast with the liquid crystal display device, the EL displaydevice is self-luminescent. The EL element has such a structure that anEL layer is sandwiched between a pair of electrodes (anode and cathode).However, the EL layer has normally a lamination structure. As a typicalexample of the lamination structures, a lamination structure “holetransport layer/light emitting layer/electron transport layer” proposedby Tang et al. of Eastman Kodak Company is cited. This structure has anextremely high light emitting efficiency. For this advantage, most lightemitting devices, which are currently under study and development,employ this structure.

Furthermore, the light emitting device may have such a laminationstructure that a hole injection layer, a hole transport layer, a lightemitting layer and an electron transport layer are deposited on an anodeor a hole injection layer, a hole transport layer, a light emittinglayer, an electron transport layer, and an electron injection layer aredeposited on an anode in this order. Moreover, the light emitting layermay be doped with a fluorescent pigment or the like.

All layers formed between a cathode and an anode are referred togenerically as EL layers within this specification. The above statedhole injecting layer, hole transporting layer, light emitting layer,electron transporting layer, electron injecting layer, and the like aretherefore all contained within the EL layer.

A predetermined voltage is then applied to the EL layer having the abovestructure by a pair of electrodes, thus recombination of a carrier thusoccurs in the light emitting layer, and light is emitted. Note that theemission of light by the EL element is referred to as driving the ELelement throughout this specification. Further, an EL element formed byan anode, an EL layer, and a cathode is referred to as an EL elementthroughout this specification.

As a method of driving an EL display device, an analog driving method(analog drive) can be given. The analog drive of an EL display device isdescribed with reference to FIGS. 10 and 11.

FIG. 10 shows a structure of a pixel portion of an EL display devicethat is driven in an analog manner. Gate signal lines (G1 through Gy) towhich a gate select signal from a gate signal line driver circuit isinput are connected to a gate electrode of a switching TFT 1801 includedin each pixel. One of a source region and a drain region of theswitching TFT 1801 included in each pixel is connected to source signallines (also referred to as data signal lines) S1 to Sx to which ananalog video signal is input, whereas the other is connected to a gateelectrode of an EL driver TFT 1804 included in each pixel and acapacitor 1808 included in each pixel.

A source region and a drain region of the driver TFT 1804 included ineach pixel are connected to power source supply lines V1 through Vx andto an EL element 1806, respectively. An electric potential of the powersource supply lines V1 through Vx is referred to as an power sourceelectric potential. The power source supply lines V1 through Vx areconnected to the capacitors 1808 included in the respective pixels.

The EL element 1806 includes an anode, a cathode and an EL layersandwiched between the anode and the cathode. If the anode of the ELelement 1806 is connected to the source or the drain region of thedriver TFT 1804, the anode and the cathode of the EL element 1806 becomea pixel electrode and an opposing electrode, respectively. On the otherhand, if the cathode of the EL element 1806 is connected to the sourceor the drain region of the driver TFT 1804, the anode and the cathode ofthe EL element 1806 become an opposing electrode and a pixel electrode,respectively.

Note that the electric potential of the opposing electrode is referredto as an opposing electric potential in this specification. Note alsothat an power source for imparting the opposing electric potential tothe opposing electrode is referred to as an opposing electric powersupply. The electric potential difference between the electric potentialof the pixel electrode and the electric potential of the opposingelectrode is an EL driver voltage, and the EL driver voltage is appliedto the EL layer.

FIG. 11 shows a timing chart in the case where the EL display deviceshown in FIG. 10 is driven in an analog manner. The period from theselection of one gate signal line until the selection of a next gatesignal line is called one line period (L). The period from the displayof one image to another image corresponds to one frame period (F). Inthe case of the EL display device shown in FIG. 10, since there are vgate signal lines, y line periods (L1 to Ly) are provided within oneframe period.

With the enhancement in resolution, the number of line periods withinone frame period increases. As a result, the driver circuit must bedriven at a high frequency.

An power source electric potential at the power source supply lines (V1through Vx) is held constant, and an opposing electric potential at theopposing electrodes is also held constant. The opposing electricpotential has a potential difference with the power source electricpotential to such a degree that a EL element 1806 emits light.

The gate signal line G1 is selected in the first line period L1 by agate signal input to the gate signal line G1 from the gate signal linedriver circuit. Then an analog video signal is then input in order tothe source signal lines S1 to Sx. All of the switching TFTs 1801connected to the gate signal line G1 are in an ON state, and thereforethe analog video signal input to the source signal lines S1 to Sx isinput to gate electrodes of the driver TFTs 1804 through the switchingTFTs 1801.

The description here takes as an example a timing chart of the casewhere the switching TFT 1801 and the driving TFT 1804 are both n-channelTFTs. The switching TFT and the driving TFT may instead be p-channelTFTs, or one of them may be an n-channel TFT while the other is ap-channel TFT.

In this specification, the TFT being turned ON means that the gatevoltage of the TFT is changed such that the source-drain thereof isbrought into conductive state.

The amount of a current flowing through a channel formation region ofthe driver TFT 1804 is controlled by a level of an electric potential(voltage) of a signal input to the gate electrode of the driver TFT1804. Accordingly, the electric potential applied to the pixel electrodeof the EL element 1806 is determined by the level of the electricpotential of the analog video signals input to the gate electrode of thedriver TFT 1804. Then, the EL element 1806 is controlled by the electricpotential of the analog video signals to emit light.

When the above-described operation is repeated to complete the input ofanalog video signals to the source signal lines (S1 through Sx), thefirst line period (L1) terminates. One line period may alternatively beconstituted by the period until the completion of input of the analogvideo signals to the source signal lines (S1 through Sx) and ahorizontal blanking period. Then, a second line period (L2) starts wherea gate signal line G2 is selected by a gate signal. And, as in the firstline period (L1), analog video signals are sequentially input to thesource signal lines (S1 through Sx) during the second line period.

When all gate signal lines (G1 through Gy) are selected in this manner,all lines periods (L1 through Ly) are completed. The completion of allthe line periods (L1 through Ly) corresponds to the completion of oneframe period. All pixels perform display during one frame period to forman image. One frame period may be alternatively constituted by all lineperiods (L1 through Ly) and a vertical blanking period.

The amount of light emitted by the EL element 1806 is thus controlled inaccordance with the analog video signal, and gray scale display isperformed by controlling the amount of light emitted. This method isnamely a driving method referred to as an analog driving method, grayscale display is performed by changing the electric potential of theanalog video signal input to the source signal lines.

In the conventional EL display device, the drain region of the drivingTFT 1804 in the pixel portion is connected only to the EL element 1806as shown in FIG. 10.

TFTs are formed on a substrate having an insulating surface in order toconstitute pixel TFTs (each formed of a switching TFT and a driving TFT)and driver circuits (including a source signal line driving circuit anda gate signal line driving circuit). An EL material is then depositedand the driving TFT is electrically connected to an EL element. Themanufacturing steps prior to the step of depositing the EL material arecalled herein TFT steps.

Before the EL material is deposited, the drain region of the driving TFTin the conventional display device is thus in an open state from thedesign of the circuit. Whether a certain pixel TFT operates normally ornot cannot be judged until the EL material is deposited to complete thedisplay device and lighting test is performed on the completed device.Therefore, it is not until after the manufacturing process reaches thefinal step that a display device incapable of normal display because ofa defective pixel TFT can be found out. This is utterly a waste.

As described above, the conventional EL display device does not allowits pixel TFTs to be checked for their operation during the steps priorto deposition of the EL material, thereby incurring a waste inmanufacturing cost.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problem, and anobject of the present invention is therefore to provide an active matrixEL display device that allows its pixel TFTs to be checked for theiroperation before depositing an EL material.

In order to attain the object above, the inventors of the presentinvention have to thought of cutting waste along the manufacture line bytesting driving TFTs and switching TFTs before depositing an EL materialand removing substrates having TFTs that are found to be defectivethrough the test (hereinafter referred to as the rejects) from thesubsequent step of depositing the EL material.

The structure of the EL display device of the present invention is asdescribed below.

In a first aspect of the present invention, an EL display device isprovided which comprises a plurality of source signal lines, a pluralityof gate signal lines, a plurality of power supply lines, a plurality ofswitching thin film transistors and a plurality of driving thin filmtransistors which are all formed on an insulating substrate, comprisinga testing capacitor which is set such that one end thereof is connectedto a drain region of each of the driving thin film transistors and theother end thereof is connected to one of the gate signal lines, andcharacterized in that the power supply lines are led out of theinsulating substrate through switches.

In a second aspect of the present invention, an EL display device isprovided characterized in that, the switches are provided for the pluralpower supply lines on one on one basis, and a driver circuit for drivingthe switches successively is placed on the insulating substrate.

In a third aspect of the present invention, an EL display device isprovided characterized in that the driver circuit for driving theswitches successively has some components shared with a source signalline driving circuit.

In a fourth aspect of the present invention, a method of testing an ELdisplay device is provided, which comprises a plurality of source signallines, a plurality of gate signal lines, a plurality of power supplylines, a plurality of switching thin film transistors and a plurality ofdriving thin film transistors which are all formed on an insulatingsubstrate, the driving thin film transistors each having a drain regionthat is connected to a testing capacitor, the method comprising thesteps of: operating the driving thin film transistors to charge thetesting capacitor until it reaches and keeps a certain level of electricpotential: turning the driving thin film transistors OFF and thensetting the electric potential of the power supply lines to a leveldifferent from the electric potential of the testing capacitor; andleading the electric charges charged in the testing capacitor out foreach pixel through its associated power supply line, so that a change inelectric potential can be detected.

In a fifth aspect of the present invention, a computer, a video camera,a head mount display, an image reproducing device, a portableinformation terminal characterized by using an EL display device.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram showing the circuit structure of a pixel portion ofa display device according to the present invention;

FIG. 2 is a block diagram of a display device according to the presentinvention;

FIG. 3 is a timing chart illustrating a method of driving a displaydevice according to the present invention:

FIG. 4 is a diagram showing an embodiment of a testing driver circuit ina display device according to the present invention;

FIG. 5 is a diagram showing an embodiment of an external testing circuitfor a display device according to the present invention;

FIGS. 6A and 6B are a top view of a display device according to thepresent invention and a sectional view thereof, respectively:

FIG. 7 is a circuit diagram showing a source signal line driving circuitof a display device according to the present invention;

FIG. 8 is a top view of a latch of a display device according to thepresent invention;

FIGS. 9A to 9E show electronic equipments employing a display device ofthe present invention;

FIG. 10 is a circuit diagram of a pixel portion of a conventionaldisplay device;

FIG. 11 is a timing chart illustrating an analog driving method for adisplay device:

FIG. 12 is a diagram showing a cellular phone that employs the presentinvention; and

FIG. 13 is a diagram illustrating how to use the cellular phone thatemploys the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The structure of an EL display device of the present invention and amethod of testing the same will be described below.

FIG. 1 shows Embodiment Mode 1 of the present invention. Shown in FIG. 1is a structural diagram of a pixel portion in the display device of thepresent invention.

Each pixel in the pixel portion comprises a power supply line (one of V1to Vx), a source signal line (one of S1 to Sx), a gate signal line (oneof G1 to Gy), a switching TFT 9102, a driving TFT 9106, a capacitorstorage 9107, an EL element 9105 and a testing capacitor 9108.

The switching TFT 9102 shown in FIG. 1 has a double gate structure.However, the switching TFT in each pixel of the display device accordingto the present invention is not limited to a double gate structure. Itmay take a single gate structure or a multi-gate structure having threeor more gates.

The driving TFT 9106 shown in FIG. 1 has a single gate structure.However, the driving TFT in each pixel of the display device accordingto the present invention may take a double gate structure or amulti-gate structure.

A drain region of the driving TFT 9106 is connected not only to the ELelement 9105 but also to the testing capacitor 9108. The testingcapacitor 9108 is interposed between the drain region of the driving TFT9106 and the gate signal line in this example. However, the structure ofthe present invention is not limited thereto and the testing capacitormay be placed between the drain region and another individual wiring.

FIG. 2 is a block diagram showing the display device of the presentinvention.

The display device comprises a source signal line driving circuit 9201,gate signal line driving circuits 9202 and 9203, a testing drivercircuit 9204, switches 9205 and 9206, source signal lines 9207 to 9209,gate signal lines 9210 to 9212, pixel portion power supply lines 9213and 9214, a power supply line lead out terminal 9215 and an externaltesting circuit 9216.

FIG. 2 shows some of the source signal lines, the gate signal lines, thepower supply lines and the switches as representatives of the rest. Thenumber of these lines actually provided corresponding to the number ofpixels that constitute the display device.

Unlike the prior art, the testing driver circuit 9204 is added to thedisplay device of the present invention. Also added are the switches9205 and 9206 that are controlled by the testing driver circuit 9204 andinterposed between the power supply line lead out terminal 9215 and thepixel portion power supply line 9213 and between the power supply linelead out terminal 9215 and the pixel portion power supply line 9214,respectively. The power supply line lead out terminal is connected tothe external testing circuit 9216.

The testing driver circuit 9204 is arranged independently in FIG. 2.However, the source signal line driving circuit may also serve as thetesting driver circuit if the signal line driving circuit is of analogtype. (Not shown)

Next, a testing method used in the present invention will be described.

The description will be given with reference to FIGS. 1 and 2.

Assumed here is a test of a substrate in a stage after the TFT steps butbefore deposition of an EL material. The test may instead be conductedat other stages as long as wire connection has already been establishedbetween the TFTs, and between the capacitors (including the capacitorstorage, the testing capacitor, etc.) and resistors or other componentswhich constitute the display device.

Note that, although the EL element 9105 is found in FIG. 1, the ELelement is not deposited and hence the EL element 9105 is not formed yetat the time of the test described below.

The first step is to apply a voltage corresponding to a signal “Hi”.e.g. 10 V. to the power supply lines. Then successively running thedriving circuits (the source signal line driving circuit 9201 and thegate signal line driving circuits 9202 and 9203), the driving TFT 9106in each pixel is turned ON to write the 10 V voltage of the power supplylines V1 to Vx in each testing capacitor 9108. Capacitance of thetesting capacitor is set from 0.05 pF to 1 pF.

The second step is as follows. The source signal line driving circuit9201 and the gate signal line driving circuits 9202 and 9203 areoperated to turn every driving TFT 9106 in the pixel portion OFF. Thenthe electric potential of the power supply lines V1 to Vx is set to avoltage corresponding to a signal “Lo”. e.g. 0 V. The switches 9205 and9206 at this point are remained ON.

The power supply lines V1 to Vx in the pixel portion thus come to have 0V.

The third step includes the following operations.

The driving TFT 9106 in each pixel is put in motion in accordance with atiming illustrated in FIG. 3.

The timing chart of FIG. 3 shows the case where the switching TFT is ann-channel TFT and the driving TFT is a p-channel TFT. However, theswitching TFT may either be an n-channel TFT or a p-channel TFT and thesame applies to the driving TFT.

The source signal lines S1 to Sx are sequentially operated. In theillustration of FIG. 3, two source signal lines S1 and S2 represent therest of the source signal lines and merely the operation for S1 and S2ill be described here. However, this operation is performed on all ofthe source signal lines S1 to Sx.

In a pixel whose source signal line has received input of a signal “Lo”,the driving TFT 9107 is turned ON when a signal “Hi” is inputted to thegate signal lines G1 to Gy successively.

If all of the power supply lines V1 to Vx are connected, wiringcapacitor is too large to detect the voltage. Therefore, in order tocheck the operation of the pixel TFTs, the switches to be connected tothe power supply lines have to be arranged such that one switch isallocated to one column of pixels.

One column of pixels here is of pixels having switching TFTs connectedto the same source signal line.

During the source signal line S1 is selected, a switch connected to thepower supply line V1 for supplying power to pixels whose switching TFTshave source regions connected to the source signal line S1 is turned ON.Meanwhile, the switches connected to the power supply lines V2 to Vxthat are associated with the other pixels are all turned OFF.

The source signal line S2 is selected next, during which a switchconnected to the power supply line V2 for supplying power to pixelswhose switching TFTs have source regions connected to the source signalline S2 is turned ON. Meanwhile, the switches connected to the powersupply lines V1 and V3 to Vx that are associated with the other pixelsare all turned OFF.

In FIG. 3, denoted by T1 and T2 are signals for turning ON or OFF theswitch connected to the power supply line V1 and the switch connected tothe power supply line V2, respectively.

In this embodiment, if T1 and T2 are signals “Hi” and inputted to powersupply lines, the switches connected to those power supply lines areturned ON. On the other hand, the switches are turned OFF if T1 and T2are signals “Lo”.

When the driving TFT 9106 is turned ON in each pixel, electric chargesheld in the testing capacitor 9108 are discharged to the power supplylines V1 to Vx. The discharge gives a voltage to the power supply linesV1 to Vx.

This voltage is given as follows. The voltage generated is expressed asV_(out) and is obtained from the equation 1:

V _(out)=10×C3/(C1+C2+C3)

where C1 is a wiring capacitance value of the power supply lines in thepixel portion. C2 is a capacitance up through the power supply line leadout terminal 9215, and C3 is a capacitance of the testing capacitor.

If C1=C2=10 pF and C3=0.1 pF, the voltage V_(out) is 0.05 V.

The voltage V_(out) is small and hence the detection thereof requiresthe external testing circuit 9216 connected to the power supply linelead out terminal 9215.

The voltage V_(out) is not generated if the pixel TFT tested isdefective because charge or discharge cannot be made successfully.

In the timing chart of FIG. 3, the voltage V_(out) is generated in thepower supply line lead out terminal 9215 every time a pixel is selectedif the pixels are normal. If the pixel TFT is defective, it can be foundout as a lack in voltage signal as indicated by 9301.

In this way, the pixel TFTs can be tested by selecting all pixels one byone.

Embodiments of the present invention will be descried below.

Embodiment 1

Embodiment 1 shows an example of the structure of a testing drivercircuit in a display device of the present invention.

In FIG. 4, the testing driver circuit comprises a shift register 9402.NAND circuits 9403, 9404 and 9405, and buffer circuits 9406, 9407 and9408. The shift register is comprised of a DFF 9401. The buffer circuitsare comprised of inverters.

Although FIG. 4 shows merely a portion of the testing driver circuitwhich corresponds to three power supply lines, the actual testing drivercircuit has all the circuits that correspond to all of the power supplylines.

The buffer circuits 9406, 9407 and 9408 have outputs 9409, 9410 and9411, respectively. The outputs are connected to switches 9205, 9206 . .. shown in FIG. 2 and further connected to pixel portion power supplylines and a power supply line lead out terminal.

When a voltage “Hi” is inputted to an input terminal 9400 of the shiftregister 9402, the terminals 9409 to 9411 all outputs signalscorresponding to “Hi”, whereby all the switches are turned ON.

Embodiment 2

Embodiment 2 shows an example of the structure of an external testingcircuit for a display device of the present invention.

In FIG. 5, an external testing circuit 9501 comprises a switch 9502 forswitching connections, an amplifier 9505 for detecting a signal, avoltage source 9503, a resistor 9504, etc.

The switch 9502 selects one connection out of connections with threeinput terminals consisting of the voltage source 9503, a voltage source9508 and the amplifier 9505. The voltage source 9503 is for a voltagecorresponding to a signal “Hi”. i.e. 10 V. The voltage source 9508 isfor a voltage corresponding to a signal “Lo”. i.e. 0 V. The amplifier9505 amplifies a signal.

Note that the voltages of the voltage sources 9503 and 9508 are notlimited to the values above but may be set to optimal values ifnecessary.

The power supply line lead out terminal of the substrate of the displaydevice to be tested is connected to an input 9507, and the test isconducted following the steps described in Embodiment Mode 1. In thetest, judgement is made by monitoring an output 9506 of the amplifier9505.

The amplifier 9505 used here has a voltage gain of about 10 to 1000folds, and amplifies a detection signal generated in the power supplyline to sense the signal. A desirable gain of the amplifier is on theorder of 100 folds.

This embodiment can be carried out in combination with Embodiment 1without restriction.

Embodiment 3

A driving TFT 108 in the present invention may be an n-channel TFT or ap-channel TFT. However, if an anode of an EL element 110 serves as apixel electrode and a cathode thereof serves as an opposite electrode, ap-channel TFT is preferably used for the driving TFT 108. On the otherhand, when the anode of the EL element 110 serves as the oppositeelectrode and the cathode thereof serves as the pixel electrode, thedriving TFT 108 is preferably an n-channel TFT.

This embodiment can be carried out in combination with Embodiment 1 or 2without restriction.

Embodiment 4

Embodiment 4 shows an example of manufacturing an EL display device inaccordance with the present invention. FIG. 6A is a top surface diagramof an EL display device using the present invention. In FIG. 6A,reference numeral 4010 denotes a substrate, 4011 denotes a pixelportion, 4012 denotes a source signal line driver circuit, and 4013 aand 4013 b denote gate signal line driver circuits. The respectivedriver circuits are connected to an external equipment via wirings 4014a, 4014 b, 4015 and 4016 leading to an FPC 4017.

Note that in this embodiment, an example where the source signal linedriver circuit 4012 functions as a testing driver circuit is described,however, the present invention is not limited to this structure. Thetesting driver circuit may be provided apart from the source signal linedriver circuit.

A cover material 6000, a sealing material (also referred to as a housingmaterial) 7000, an airtight material (a second sealing material) 7001are provided at this time so as to surround at least the pixel portion,and preferably the driver circuit and the pixel portion.

Further, FIG. 6B is a cross sectional structure of an EL display deviceof this embodiment, and a driver circuit TFT (note that a CMOS circuitin which an n-channel TFT and a p-channel TFT are combined is shown inthe figures here) 4022 and a pixel portion TFT 4023 (note that only adriver TFT for controlling the electric current to the EL element isshown in the figures here) are formed on a base film 4021 on thesubstrate 4010. Known structures (top gate structures or bottom gatestructures) may be used for these TFTs.

Note that a testing capacitor connected to a drain electrode of thedriver TFT is not shown in FIG. 6B.

After completing the driver circuit TFT 4022 and the pixel portion TFT4023, a pixel electrode 4027 made from a transparent conductive film forelectrically connecting to a drain of the pixel portion TFT 4023 isformed on an interlayer insulating film (leveling film) 4026 made from aresin material. A compound of indium oxide and tin oxide (referred to asITO) and a compound of indium oxide and zinc oxide can be used as thetransparent conductive film. An insulating film 4028 is formed once thepixel electrode 4027 is formed, and an open portion is formed on thepixel electrode 4027.

Then, an EL layer 4029 is formed. A lamination structure of a known ELmaterial (hole injecting layer, hole transporting layer, light emittinglayer, electron transporting layer, and electron injecting layer), or asingle layer structure, may be used for the EL layer 4029. The structureis formed using a known technique. Further, there are low molecularweight materials and high molecular weight materials (polymer materials)for the EL material. An evaporation method is used when a low molecularweight material is used, but it is possible to use a simple method suchas spin coating, printing or ink-jet printing when a high molecularweight material is used.

The EL layer 4029 is formed by evaporation using a shadow mask in thisembodiment. Color display becomes possible by forming light emittinglayers (a red color light emitting layer, a green color light emittinglayer, and a blue color light emitting layer) capable of emitting lightat different wavelength for each pixel using the shadow mask. Inaddition, a method of combining a color changing layer (CCM) and a colorfilter, and a method of combining a white color light emitting layer anda color filter are available, and both may be used. Of course, a singlecolor light emitting EL display device can also be made.

After forming the EL layer 4029, a cathode 4030 is formed on the ELlayer. It is preferable to remove as much moisture and oxygen aspossible from the interface between the cathode 4030 and the EL layer4029. Therefore, a method in which the EL layer 4029 and the cathode4030 are formed in succession within a vacuum, or in which the EL layer4029 is formed in an inert atmosphere and the cathode 4030 is thenformed without exposure to the atmosphere is necessary. In thisembodiment, the above film formation can be performed by using amulti-chamber method (cluster tool method) film formation apparatus.

Note that a lamination structure of a LiF (lithium fluoride) film and anAl (aluminum) film is used as the cathode 4030 in this embodiment.Specifically, a 1 nm thick LiF (lithium fluoride) film is formed byevaporation on the EL layer 4029, and a 300 nm thick aluminum film isformed on the LiF film. An MgAg electrode, which is a known cathodematerial, may of course also be used. Then, the cathode 4030 isconnected to the wiring 4016 in a region denoted by reference numeral4031. The wiring 4016 is an electric power source supply line forapplying a predetermined voltage to the cathode 4030, and is connectedto the FPC 4017 through a conductive paste material 4032.

The cathode 4030 and the wiring 4016 are electrically connected in theregion shown by reference numeral 4031, and therefore it is necessary toform contact holes in the interlayer insulating film 4026 and theinsulating film 4028. These contact holes may be formed during etchingof the interlayer insulating film 4026 (when the pixel electrode contacthole is formed) and during etching of the insulating film 4028 (whenforming the open portion before forming the EL layer). Further, etchingmay also be performed together through to the interlayer insulating film4026 when etching the insulating film 4028. A contact hole having a goodshape can be formed in this case provided that the interlayer insulatingfilm 4026 and the insulating film 4028 are formed by the same resinmaterial.

A passivation film 6003, a filling material 6004, and the cover material6000 are formed covering the surface of the EL element thus formed.

In addition, the sealing material 7000 is formed on the inside of thecover material 6000 and the substrate 4010 so as to surround the ELelement portion. The airtight material (the second sealing material)7001 is formed on the outside of the sealing material 7000.

The filling material 6004 functions as an adhesive for bonding the covermaterial 6000. PVC (polyvinyl chloride), epoxy resin, silicone resin,PVB (polyvinyl butyral) and EVA (ethylene vinyl acetate) can be used asthe filling material 6004. A moisture absorption effect can bemaintained if a drying agent is formed on the inside of the fillingmaterial 6004, and therefore it is preferable to do so.

Furthermore, spacers may be included within the filling material 6004.The spacers may be made from a powdered substance comprising a materialsuch as BaO, giving the spacers themselves moisture absorbency.

In the case of providing the spacers, the passivation film 6003 canrelieve the spacer pressure. Further, a film such as a resin film,separate from the passivation film 6003, may also be formed forrelieving the spacer pressure.

Further, a glass plate, an aluminum plate, a stainless steel plate, anFRP (fiberglass-reinforced plastics) plate, a PVF (polyvinyl fluoride)film, a mylar film, a polyester film, and an acrylic film can be used asthe cover material 6000. Note that when using PVB or EVA as the fillingmaterial 6004, it is preferable to use a sheet having a structure inwhich several 10 of μm of aluminum foil is sandwiched by a PVF film or amylar film.

Note that, depending upon the direction of light emitted from the ELelements (light emission direction), it may be necessary for the covermaterial 6000 to have light transmitting characteristics.

Further, the wiring 4016 is electrically connected to the FPC 4017through a gap between the sealing material 7000 and the alright material7001, and the substrate 4010. Note that, although the wiring 4016 isexplained here, the other wirings 4014 a, 4014 b, and 4015 are alsoelectrically connected to the FPC 4017 through a gap between the sealingmaterial 7000 and the airtight material 7001, and the substrate 4010.

Note that the cover material 6000 is bonded after forming the fillingmaterial 6004 in Embodiment 5, and that the sealing material 7000 isattached so as to the side surface (exposed surface) of the fillingmaterial 6004, but the filling material 6004 may also be formed afterattaching the cover material 6000 and the sealing material 7000. Afilling material injection port passing through the gap formed by thesubstrate 4010, the cover material 6000 and the sealing material 7000 isformed in this case. The gap is then placed in a vacuum state (equal toor less than 10⁻² Torr), and after immersing the injection port in atank containing the filling material, the pressure on the outside of thegap is made higher than the pressure within the gap, and the fillingmaterial fills the space.

This embodiment can be carried out in combination with Embodiments 1 to3 without restriction.

Embodiment 5

Embodiment 5 describes the structure of a source signal line drivingcircuit in the case of employing a digital time gray scale drivingmethod instead of the analog gray scale driving method that has beendescribed in the example of the prior art.

FIG. 7 shows as a circuit diagram an example of the source signal linedriving circuit used in this embodiment.

The present invention can adopt any one of the analog gray scale drivingmethod, the digital time gray scale driving method and the digital arearatio gray scale driving method. The invention may also take a drivingmethod that uses these gray scale methods in combination.

The source signal line driving circuit has a shift register 801, latches(A) 802 and latches (B) 803 which are arranged as shown in FIG. 7.

In this embodiment, a pair of latches (A) 802 and a pair of latches (B)803 are associated with outputs to four source signal lines S_a to S_d.Therefore, four input lines VD are provided for digital image signalsthat are inputted from the external. The four input lines VDrespectively receive signals to be inputted to the source signal linesS_a to S_d.

A level shifter for changing the amplitude of the voltage of the signalis not provided in this embodiment. However, the level shifter may beprovided at discretion.

A clock signal CLK, an inverted clock signal CLKB obtained by invertingthe polarity of the CLK, a start pulse signal SP, and a drive directionswitching signal SL/R are inputted to the shift register 801respectively from the wirings shown in FIG. 7. A digital data signal VDis inputted to the latches (A) 802 from the wirings shown in FIG. 7. Alatch signal S_LAT and an inverted signal S_LATb obtained by invertingthe polarity of the S_LAT are inputted to the latches (B) 803respectively from the wirings shown in FIG. 7.

Details of the structure of the latches (A) 802 will be described takingas an example a portion 804 that is a part of the latches (A) 802associated with the source signal line S_a. The portion 804 that is apart of the latches (A) 802 has two clocked inverters and two inverters.

FIG. 8 shows a top view of the portion 804 that is a part of the latches(A) 802. Denoted by 831 a and 831 b are active layers of TFTs thatconstitute one of the inverters of the portion 804 that is a part of thelatches (A) 802. Reference symbol 836 denotes a common gate electrode ofthe TFTs constituting the one inverter. The other inverter of theportion 804 that is a part of the latches (A) 802 comprises TFTs whoseactive layers are denoted by 832 a and 832 b. On the active layers 832 aand 832 b. gate electrodes 837 a and 837 b are provided. The gateelectrodes 837 a and 837 b are electrically connected to each other.

Denoted by 833 a and 833 b are active layers of TFTs that constitute oneof the clocked inverters of the portion 804 that is a part of thelatches (A) 802. On the active layer 833 a, gate electrodes 838 a and838 b are formed to provide a double gate structure. On the active layer833 b, the gate electrode 838 b and a gate electrode 839 are formed toprovide a double gate structure.

Denoted by 834 a and 834 b are active layers of TFTs that constitute theother clocked inverter of the portion 804 that is a part of the latches(A) 802. On the active layer 834 a. the gate electrode 839 and a gateelectrode 840 are formed to provide a double gate structure. On theactive layer 834 b. the gate electrode 840 and a gate electrode 841 areformed to provide a double gate structure.

This embodiment can be carried out in combination with any one ofEmbodiments 1 through 4 without restriction.

Embodiment 6

According to an EL display device of the present invention, a materialused for an EL layer of an EL element is not limited to an organic ELmaterial but may be an inorganic EL material. However, every inorganicEL material at present is very high in drive voltage and hence a TFT tobe used has to have a withstand voltage characteristic that allows theTFT to withstand such a high drive voltage.

If an inorganic EL material of lower drive voltage is to be developed infuture, it may also be used in the present invention.

This embodiment can be carried out in combination with any one ofEmbodiments 1 through 5 without restriction.

Embodiment 7

An electronic display formed by implementing the present invention, inparticular, an EL display device is utilized for various electronicequipments. Electronic equipments incorporating an electronic displayformed in accordance with this invention as a display medium will beexplained as follows.

The following can be given as examples of such electronic equipments: avideo camera; a television receiving machine: a digital camera, a headmounted display (a goggle type display); a game machine; a telephone; acar navigation system: a personal computer; an image reproducing device;a portable information terminal (such as a mobile computer, a mobiletelephone, or an electronic book). Examples of those electronicequipments are shown in FIG. 9.

FIG. 9A illustrates a personal computer, which includes a main body2001, a frame 2002, a display portion 2003, a key board 2004, or thelike. The EL display device of the present invention can be applied tothe display portion 2003 of the personal computer.

FIG. 9B illustrates a video camera, which includes a main body 2101, adisplay portion 2102, an audio input portion 2103, operation switches2104, a battery 2105, an image receiving portion 2106, or the like. TheEL display device of the present invention can be applied to the displayportion 2102 of the video camera.

FIG. 9C illustrates a portion (the right-half piece) of a head mounteddisplay, which includes a main body 2301, signal cables 2302, a headmount band 2303, a display monitor 2304, an optical system 2305, adisplay portion 2306, or the like. The EL display device of the presentinvention can be applied to the display portion 2306 of the head mounteddisplay.

FIG. 9D shows an image reproducing device provided with a recordingmedium (specifically, a DVD reproducing device). The image reproducingdevice comprises a main body 2401, a recording medium (CD, LD, DVD, orthe like) 2402, operation switches 2403, a display portion (a) 2404, adisplay portion (b) 2405, etc. The display portion (a) mainly displaysimage information whereas the display portion (b) mainly displayscharacter information. The EL display device of the present inventioncan be used for the display portions (a) and (b) of the imagereproducing device provided with a recording medium. Note that thepresent invention also can be applied to other image reproducingmachines provided with a recording medium, such as CD players and gamemachines.

FIG. 9E shows a portable (mobile) computer, which includes a main body2501, a camera portion 2502, image receiving portion 2503, operationswitches 2504, a display portion 2505, or the like. The EL displaydevice of the present invention can be applied to the display portion2505 of the portable (mobile) computer.

If the luminance of light emitted from EL materials is improved infuture, the present invention can be used in front or rear typeprojectors.

The electronic equipments in this embodiment can be carried out incombination with any one of Embodiments 1 through 6 without restriction.

Embodiment 8

FIG. 12 shows an example in which an EL display device is used in acellular phone.

The cellular phone comprises a casing A 1201, a casing B 1202 and anantenna 1205. A display portion 1200 and a microphone 1209 are formed ona surface A 1203 of the casing A 1201. Formed on a surface B 1204 of thecasing B 1204 are a speaker 1206, operation keys 1207, a power switch1208, etc.

An EL display device of the present invention can be used as the displayportion 1200 of the cellular phone.

The arrangement of the speaker 1206, the operation keys 1207, thedisplay portion 1200, the microphone 1209 and the power switch 1208 isnot limited to the one described above. The components can be formed onany part of the casing A 1201 and the casing B 1202.

In FIG. 12, the cellular phone has two casing portions (the casing A1201 and the casing B 1202) connected by a hinge (not shown) at oneside. When the hinge is closed, the surface A 1203 of the casing A 1201can be laid on top of the surface B 1204 of the casing B 1202. To laythe surface A 1203 of the casing A 1201 on top of the surface B 1204 ofthe casing B 1202 is referred to as folding the cellular phone in ishalf.

In this example, the cellular phone can be used in a way illustrated inFIG. 13. Since the speaker 1206 and the microphone 1209 are set indifferent casings, it is possible to place the speaker 1206 close to aear 1211 and the microphone 1209 to a mouth 1212 with an adjustment ofthe angle between the surface A 1203 and the surface B 1204 on whichthey are set. This structure has an advantage of blocking the view ofthe mouth 1212 from the others while the user is speaking on the phone.Moreover, the microphone 1209 being in proximity to the mouth 1212reduces the influence of noise and thereby improves the phonecommunication quality. Thus it is effective in reducing the number ofnoise filters in the cellular phone. With more operation keys 1207, thecellular phone can also serve as a portable information terminal.

As described above, the application range of the present invention is sowide that the invention can be applied to every field of electronicequipment. The electronic equipment of this embodiment can be obtainedusing any combination of structures of Embodiments 1 to 6.

With the structure above, a display device that allows its pixel TFTs tobe tested before forming an EL layer can be provided. This makes itpossible to remove the rejects before depositing an EL material, leadingto a reduction in manufacturing cost.

1. A display device comprising: a first transistor; a second transistor;a first capacitor; a pixel electrode; a first line; a second line; athird line; and a fourth line; wherein a gate of the first transistor iselectrically connected to the third line, one of a source and a drain ofthe first transistor is electrically connected to the first line, theother of the source and the drain of the first transistor iselectrically connected to a gate of the second transistor, one of asource and a drain of the second transistor is electrically connected tothe second line, the other of the source and the drain of the secondtransistor is electrically connected to the pixel electrode and a firstterminal of the first capacitor, and a second terminal of the firstcapacitor is electrically connected to the fourth line, and wherein aninsulating film is formed over the pixel electrode and the fourth line,and an opening is provided in the insulating film at the pixelelectrode.
 2. The display device according to claim 1, furthercomprising a second capacitor, wherein a first terminal of the secondcapacitor is electrically connected to the gate of the secondtransistor, and a second terminal of the second capacitor iselectrically connected to the second line.
 3. The display deviceaccording to claim 1, wherein the first transistor is an n-channel typetransistor, and the second transistor is a p-channel type transistor. 4.The display device according to claim 1, wherein the first transistorand the second transistor are n-channel type transistors.
 5. The displaydevice according to claim 1, wherein the first transistor and the secondtransistor are polysilicon thin film transistors.
 6. The display deviceaccording to claim 1, wherein the first transistor and the secondtransistor are amorphous silicon thin film transistors.
 7. The displaydevice according to claim 1, wherein the third line and the fourth lineare arranged parallel to each other.
 8. The display device according toclaim 1, wherein the third line and the fourth line are scanning lines.9. The display device according to claim 1, wherein the first transistoris a multi-gate type transistor.
 10. The display device according toclaim 1, further comprising an organic light emitting layer formed overthe pixel electrode.
 11. The display device according to claim 10,wherein the first transistor, the second transistor and the organiclight emitting layer are formed over a first substrate, and covered witha second substrate.
 12. The display device according to claim 11,wherein the second substrate has light transmitting characteristics, andlight emitted from the organic light emitting layer passes through thesecond substrate to display images.
 13. The display device according toclaim 10, further comprising a passivation film formed over the organiclight emitting layer.
 14. The display device according to claim 10,further comprising a resin layer formed over the organic light emittinglayer.
 15. The display device according to claim 10, further comprisinga color filter.
 16. The display device according to claim 12, whereinthe resin layer comprises a drying agent.
 17. A display devicecomprising: a first n-channel type transistor; a second n-channel typetransistor; a first capacitor; a pixel electrode; a first line; a secondline; a third line; and a fourth line; wherein a gate of the firstn-channel type transistor is electrically connected to the third line,one of a source and a drain of the first n-channel type transistor iselectrically connected to the first line, the other of the source andthe drain of the first n-channel type transistor is electricallyconnected to a gate of the second n-channel type transistor, one of asource and a drain of the second n-channel type transistor iselectrically connected to the second line, the other of the source andthe drain of the second n-channel type transistor is electricallyconnected to the pixel electrode and a first terminal of the firstcapacitor, and a second electrode of the first capacitor is electricallyconnected to the fourth line, and wherein an insulating film is formedover the pixel electrode and the fourth line, and an opening is providedin the insulating film at the pixel electrode.
 18. The display deviceaccording to claim 17, further comprising a second capacitor, wherein afirst terminal of the second capacitor is electrically connected to thegate of the second n-channel type transistor, and a second terminal ofthe second capacitor is electrically connected to the second line. 19.The display device according to claim 17, wherein the first n-channeltype transistor and the second n-channel type transistor are polysiliconthin film transistors.
 20. The display device according to claim 17,wherein the first n-channel type transistor and the second n-channeltype transistor are amorphous silicon thin film transistors.
 21. Thedisplay device according to claim 17, wherein the third line and thefourth line are arranged parallel to each other.
 22. The display deviceaccording to claim 17, wherein the third line and the fourth line arescanning lines.
 23. The display device according to claim 17, whereinthe first n-channel type transistor is a multi-gate type transistor. 24.The display device according to claim 17, further comprising an organiclight emitting layer formed over the pixel electrode.
 25. The displaydevice according to claim 24, wherein the first transistor, the secondn-channel type transistor and the organic light emitting layer areformed over a first substrate, and covered with a second substrate. 26.The display device according to claim 25, wherein the second substratehas light transmitting characteristics, and light emitted from theorganic light emitting layer passes through the second substrate todisplay images.
 27. The display device according to claim 24, furthercomprising a passivation film formed over the organic light emittinglayer.
 28. The display device according to claim 24, further comprisinga resin layer formed over the organic light emitting layer.
 29. Thedisplay device according to claim 24, further comprising a color filter.30. The display device according to claim 28, wherein the resin layercomprises a drying agent.